Sabertooths do not attack player neither day nor night. They attack only if a player attacks first. If i'm not mistaken sabertooths should attack players on day as an opposite to panther that attacks on night.

Version: TerraFirmaCraft MC1.12.2-1.5.4.154

I'm very tempted to say this is actually just due to #1480 and that the sabertooths you're seeing are just children, and hence not aggressive. (I'm not aware if they are meant to still be aggressive as children, @DisasterMoo can probably answer that.

Sabertooth attack at night, Lions are the only current Feline family that attack during the day. Will be changed to match other Felines in upcoming update

Second to Abdyman: yes, Sabertooth's AI works like the other predators and only attack durning the night while children won't do anything until provoked, most certainly it was related to #1480.